Nuclear fuel undergoes fission to produce energy in a nuclear reactor, and is a very high-density energy source. Solid pellets of oxide fuels such as uranium dioxide are commonly used in today's reactors because they are relatively simple and inexpensive to manufacture, can achieve high effective uranium densities and have a high melting point. They also provide well-established pathways to reprocessing. For example, solid uranium dioxide (“UO2”) is widely used in CANada Deuterium Uranium (“CANDU”) reactor and other reactors. To be used as a fuel, uranium dioxide is compacted into cylindrical pellets and sintered at high temperatures to produce ceramic nuclear fuel pellets with a high density. Such fuel pellets are then stacked into metallic tubes (“cladding”). Cladding prevents radioactive fission fragments from escaping from the fuel into the coolant and contaminating it. The metal used for the tubes depends on the design of the reactor. Stainless steel was used in the past, but most reactors now use a zirconium alloy which, in addition to being highly corrosion-resistant, has low neutron absorption.
The sealed tubes containing the fuel pellets are termed fuel rods, which are grouped into fuel assemblies used to build up the core of a nuclear power reactor. Each fuel assembly includes fuel rods bundled in an arrangement of 16×16 or 17×17 in current Pressurized Water Reactors (“PWRs”) depending on the reactor core design. A reactor core includes multiple fuel assemblies, such as 400 to 800 fuel assemblies. In CANDU reactors the fuel rods are arranged in cylindrical canisters, each containing 20-40 short fuel rods depending on the design. There are hundreds of canisters lined up in tubes with coolant and moderator and they are moved in and out of the reactor at a predetermined rate.
Tristructural-isotropic (“TRISO”) fuel particles compacted within a graphite matrix have been developed for a new generation of gas-cooled reactors. A TRISO fuel particle comprises a kernel of fissile/fertile material coated with several isotropic layers of pyrolytic carbon (“PyC”) and silicon carbide (“SiC”). These TRISO particles are combined with a graphite matrix material and pressed into a specific shape. The TRISO fuel forms offer much better fission product retention at higher temperatures and burnup than metallic or solid oxide fuel forms.
Burnup is a measure of how much energy is extracted from a nuclear fuel source. It is measured as the fraction of fuel atoms that underwent fission in fissions per initial metal atom (“FIMA”). Burnup is also measured as the actual energy released per mass of initial fuel in, for example, megawatt-days/kilogram of heavy metal (“MWd/kgHM”). Higher burnup may not only reduce the overall waste volume but also limit possible nuclear proliferation and diversion opportunities. While high burnup is desirable, it is also important that burnup rates for the replacement TRISO based fuel should be not too fast and should at least match the burnup rate of the reference standard fuel, in order to achieve comparable service life in the reactor.
Recently, the fully ceramic micro-encapsulated (“FCM”) fuel has been proposed, wherein FCM fuel utilizes TRISO fuel particles, which are pressed into compacts using SiC matrix material and loaded into fuel pins. However, the heavy metal mass in a FCM fuel pellet tends to be considerably lower than that of a conventional solid fuel pellet due to the limited space available for heavy metal and fissile mass inside TRISO particles of FCM fuel. The heavy metal and fissile mass in a FCM fuel pellet can be increased, by increasing the diameter of the FCM fuel pellet or the kernel diameter (“KD”) of TRISO particles within the FCM fuel pellet or the packing fraction of the TRISO particles, or using a high density material in the kernel, such as Uranium Nitride or Uranium Silicide. Under the first approach, for example, 12×12 FCM fuel assemblies replace conventional 16×16 solid fuel assemblies and 13×13 FCM fuel assemblies replace conventional 17×17 solid fuel assemblies. Under the latter approach, the kernel diameter of TRISO particles can be increased to, for example, 400 μm, 500 μm, 600 μm, 700 μm, or 800 μm.
The reactivity characteristics of a FCM fuel replacement assembly depends on the type(s) of material and enrichment comprising the kernel of TRISO particles. Accordingly, there exists a need for a FCM fuel assembly to achieve reactivity characteristics that are comparable to or better than that of a standard reference reactor fuel assembly, such as the widely used solid UO2 assembly.